1. Field of the Invention The present invention relates to a transparent conductive layered structure having a transparent substrate and a transparent conductive layer and transparent coating layer formed in succession on this substrate, which is used in front panels of display devices such as CRT, etc. The present invention particularly relates to a transparent conductive layered structure with excellent weather resistance, ultraviolet ray resistance, conductivity, etc., and with which a reduction in production cost is expected and a method of producing the same, and a coating liquid for forming a transparent conductive layer used in the production of a transparent conductive layered structure and a method of producing the same.
2. Description of the Prior Art
Many OA devices have been introduced to the office as a result of office automation (OA) in recent years and an environment in which the entire day work must be done facing the display of an OA device is no longer uncommon.
However, when a job is done next to a cathode ray tube (CRT) of a computer, etc., as an example of OA equipment, it must be easy to see the display screen in order to prevent visual fatigue, as well as prevent deposition of dust and electric shock induced by the electrostatic charge on the CRT screen, etc. Furthermore, in addition to these requirements, etc., there has recently been concern over the detrimental effects of low-frequency electromagnetic waves generated by CRTs on the human body and there is a demand for CRTs with which there is no leakage to the outside of such electromagnetic waves.
The above-mentioned electromagnetic waves are generated from deflecting coils and fly-back transformers and the development of larger televisions has led to a tendency toward leakage of increasingly larger amounts of electromagnetic waves around televisions.
For the most part, leakage of a magnetic field can be prevented by techniques such as changing the shape of the deflecting coil, etc. On the other hand, it is also possible to prevent leakage of an electric field by coating the front glass surface of a CRT with a transparent conductive layer.
Such methods for preventing leakage of an electric field are theoretically the same as measures that have been adopted in recent years to prevent electrostatic charging. However, the conductivity of the above-mentioned transparent conductive layer must be much higher than that of conductive layers that are formed to prevent electrostatic charging. That is, although surface resistance of 108 xcexa9/xe2x96xa1 (ohm per square) is enough to prevent electrostatic charging, a transparent conductive layer with at least as low a resistance as 106 xcexa9/xe2x96xa1 or less, preferably 103 xcexa9/xe2x96xa1 or less, is preferred in order for prevention of leakage of an electric field (electric field shielding).
Therefore, several suggestions have been made thus far for meeting the above-mentioned requirements, but of these, the method wherein a coating liquid for forming a transparent conductive layer of conductive microparticles dispersed with inorganic binder, such as alkyl silicate, etc., in a solvent is applied to the front glass of a CRT and dried and then baked at a temperature of 200xc2x0 C. is known as a method with which low cost and low surface resistance can be realized.
In addition, this method that uses a coating liquid for forming a transparent conductive layer is very simple when compared to other methods of forming transparent conductive layers, such as vacuum evaporation and sputtering, has a low production cost, and is a very useful method in terms of electric field shielding treatment of CRTs.
It is known that the above-mentioned coating liquid that is used to form a transparent conductive layer used by this method employs indium tin oxide (ITO) as the conductive microparticles. However, because surface resistance of the film that is obtained is high at 104 to 106 xcexa9/xe2x96xa1, a corrective circuit for canceling the electric field is needed in order to sufficiently block leakage of an electric field. Therefore, there has been a problem in that production cost rises accordingly. On the other hand, when compared to coating liquids that use ITO, a film with somewhat lower transmittance, but also low resistance of 102 to 103 xcexa9/xe2x96xa1, is obtained with coating liquids for forming transparent conductive layers that use metal powder for the above-mentioned conductive microparticles. Consequently, there is an advantage in terms of cost because the above-mentioned correcting circuit is not necessary, and this will probably be the prevailing method of the future.
Moreover, the metal microparticles that are used in the above-mentioned coating liquid for forming the above-mentioned transparent conductive layer are limited to noble metals, such as silver, gold, platinum, rhodium, palladium, etc., that rarely oxidize in air, as shown in Japanese Patent Applications Laid-Open No. H 8-77832 and Laid-Open No. H 9-55175. This is because if microparticles of a metal other than a noble metal, such as iron, nickel, cobalt, etc., are used, an oxide film is invariably formed on the surface of such metal microparticles in an air atmosphere and good conductivity cannot be obtained as a transparent conductive layer.
Moreover, on the other hand, in order to make the display screen easy to see, anti-glare treatment is performed on the face panel surface to prevent reflection on the screen. This antiglare treatment is performed by the method whereby fine irregularities are made in the surface in order to increase diffused reflection at the surface, but it cannot be said that this method is a very desirable method because when used, resolution decreases and picture quality drops. Consequently, it is preferred that antiglare treatment be performed by the interference method whereby the refractive index and film thickness of the transparent film be controlled so that there is destructive interference of the incident light by the reflected light. A two-layered film structure wherein optical film thickness of film with a high refractive index and film with a low refractive index has been set at xc2xcxcex and xc2xcxcex (xcex is wavelength), or xc2xdxcex and xc2xcxcex, respectively, is usually used in order to obtain this type of low-reflection effect of the interference method, and film consisting of the above-mentioned indium tin oxide (ITO) microparticles is also used as this type of film with a high refractive index.
Furthermore, of the parameters that make up the optical constant (n-ik, n: refractive index, i2=xe2x88x921, k: extinction coefficient) of metals, the value of n is small, but the value of k is very large when compared to ITO and therefore, even if a transparent conductive layer consisting of metal microparticles is used, the same anti-reflection activity induced by interference of light as seen with ITO is obtained with the two-layered film structure.
However, as previously mentioned, the metal microparticles used in conventional coating liquid for forming a transparent conductive layer are limited to noble metals such as silver, gold, platinum, rhodium, palladium, etc. Nevertheless, when the specific resistance of these is compared, resistivity of platinum, rhodium, and palladium is 10.6, 5.1, and 10.8 xcexcxcexa9xc2x7cm, respectively, which is high when compared to the 1.62 and 2.2 xcexcxcexa9xc2x7cm of silver and gold. Therefore, it is more of an advantage to use silver microparticles and gold microparticles to form a transparent conductive layer with low surface resistance.
There was, however, a problem with weather resistance in that there was severe deterioration due to sulfurization, oxidation, and exposure to ultraviolet rays and brine, when silver microparticles were used, while when gold microparticles were used, there were none of the above-mentioned problems with weather resistance, but there were the same problem with cost as when platinum microparticles, rhodium microparticles, palladium microparticles, etc., were used. Furthermore, the use of gold microparticles posed a problem in that because the transparent conductive layer itself that was formed adsorbed some of the visible light rays due to optical properties inherent to metals, it could not be used for the display surface of displays such as CRTs, etc., which require a flat transmitted light profile within the entire visible light region.
In light of this related art, the inventors previously presented a coating liquid for forming a transparent conductive layer in which are dispersed noble metal-coated silver microparticles with a mean particle diameter of 1 to 100 nm, wherein the surface of the silver microparticles is coated with gold or platinum only or a compound of gold and platinum in place of the above-mentioned silver or gold microparticles, and a transparent conductive layered structure produced using this coating liquid, as well as a display that uses this layered structure, etc. (refer to each Specification of Japanese Patent Applications No. H9-309350, No. H9-309351, No. H9-332400, and No. H9-332401).
Moreover, improvement of weather resistance, chemical resistance, etc., is expected when the surface of the silver microparticles is coated with gold or platinum only or a compound of gold and platinum because the silver inside the noble-metal coated silver microparticles is protected by the gold or platinum only or by the compound of gold and platinum.
That is, the above-mentioned transparent conductive layer of the transparent conductive layered structure is obtained by applying a coating liquid for forming a transparent conductive layer in which noble metal-coated silver microparticles have been dispersed on a transparent substrate and a successive heat treatment. Moreover, by means of this heat treatment, each of the noble metal-coated silver microparticles bond together with the gold or platinum only or compound of gold and platinum remaining coated on the surface of the silver microparticles, constituting noble metal microparticles comprising gold and/or platinum and silver. Therefore, the silver inside the noble metal microparticles is protected by gold or platinum only, or by a compound of gold and platinum, and improvement of weather resistance, chemical resistance, etc., of the noble metal microparticles in the transparent conductive layer is expected.
Furthermore, the inventors also studied methods of making alloy particles by alloying the silver with gold or platinum, or gold and platinum, and thereby improving properties, such as the above-mentioned weather resistance, etc., in place of the above-mentioned method whereby gold or platinum only or a compound of gold and platinum is coated on the surface of silver microparticles.
However, when an aqueous solution of chloroaurate or chloroplatinate and silver salt is used as the starting solution for making the above-mentioned alloy microparticles by the wet method, which is commonly used for microparticle preparation, there is a problem in that sparingly-soluble silver chloride is produced when these are mixed. Moreover, although the above-mentioned problem is not produced when a cyanogen complex salt is used for the gold salt, platinum salt and silver salt, there is a problem in that it becomes necessary to handle toxic cyanogen compounds and synthesis of alloy microparticles of the gold or platinum and silver is not easy.
Therefore, the inventors planned to not use the latter method and to solve the above-mentioned conventional problems by the former method that uses noble metal-coated silver microparticles.
However, depending on the conditions of heat treatment after applying the coating liquid for forming a transparent conductive layer on a transparent substrate of the former method that uses noble metal -coated silver microparticles, an alloyed layer is made wherein part of the silver is diffused to inside the noble metal coating layer formed from gold and/or platinum and some of this alloyed layer is exposed at the surface.
In addition, this alloyed layer poses problems in that because there is deterioration of chemical stability when compared to the noble metal coating layer formed from gold and/or platinum, there is accordingly a slight drop in weather resistance, ultraviolet ray resistance, chemical resistance, etc., and this becomes particularly obvious with an increase in the percentage of silver in the above-mentioned alloyed layer.
The present invention focuses on such problems, its object being to present a transparent conductive layered structure with which there is rarely a reduction in weather resistance, ultraviolet ray resistance, etc., even when the heat treatment conditions during production are set as needed.
Another object of the present invention is to present a production method for obtaining a transparent conductive layered structure with excellent weather resistance, ultraviolet ray resistance, conductivity, etc., even when the heat treatment conditions during production are set as needed.
Yet another object of the present invention is to present a coating liquid for forming a transparent conductive layer that is used in the production of a transparent conductive layered structure with excellent weather resistance, ultraviolet ray resistance, conductivity, etc.
Still another object of the present invention is to present a method of producing the above-mentioned coating liquid for forming a transparent conducive layer.
That is, the present invention is a transparent conductive layered structure comprising a transparent substrate, a transparent conductive layer; and a transparent coating layer, wherein said transparent conductive layer and transparent coating layer are formed in succession on said transparent substrate, and the main components of said transparent conductive layer are noble metal microparticles made from gold and/or platinum and silver with a mean particle diameter of 1 to 100 nm and a gold and/or platinum content within a range exceeding 50 wt % up to 95 wt % and binder matrix.
Moreover, the method of producing the transparent conductive layered structure of the present invention comprises the steps of applying a coating liquid for forming a transparent conductive layer on said transparent substrate, then applying a coating liquid for forming the transparent coating layer; and performing heat treatment, with the main components of said coating liquid for forming a transparent conductive layer being noble metal-coated silver microparticles with a mean diameter of 1 to 100 nm, wherein the surface of silver microparticles is coated with gold or platinum only or a compound of gold and platinum and the content of gold and/or platinum is within a range exceeding 50 wt % up to 95 wt %, and a solvent that will disperse these microparticles.
Next, the coating liquid for forming a transparent conductive layer used to produce the above-mentioned transparent conductive layered structure comprises as its main components solvent and noble metal-coated silver microparticles with a mean particle diameter of 1 to 100 nm dispersed in the solvent, wherein the surface of the silver microparticles is coated with gold or platinum only or a compound of gold and platinum and the gold and/or platinum content is within a range exceeding 50 wt % up to 95 wt %.
In addition, the method of producing this coating liquid for forming a transparent conductive layer comprises the steps of adding reducing agent and alkali metal aurate solution and/or alkali metal platinate solution, or said reducing agent and mixed solution of alkali metal aurate and platinate, to a colloidal dispersion of silver microparticles and adjusting each mixture ratio of the colloidal dispersion of silver microparticles and alkali metal aurate solution and/or alkali metal platinate solution, or colloidal dispersion of silver microparticles and mixed solution of alkali metal aurate and alkali metal platinate, to obtain a colloidal dispersion of noble metal-coated silver microparticles having a gold and/or platinum content within a range exceeding 50 wt % up to 95 wt % and thereby prepare noble metal-coated silver microparticles by a noble metal-coated silver microparticle preparation process, desalting in order to reduce the electrolyte concentration of said colloidal dispersion of noble metal-coated silver microparticles and concentrating in order to concentrate said colloidal dispersion and thereby obtain a concentrated dispersion of noble metal-coated silver microparticles by a desalting and concentrating process, and adding and mixing solvent only, or solvent comprising conductive oxide microparticles and/or inorganic binder, with said concentrated dispersion of noble metal-coated silver microparticles to obtain a coating liquid for forming a transparent conductive layer by a solvent mixing process.